The last four decades have seen an unparalleled impact on semiconductor industry generated by silicon technology, accounting for over 90% of the global semiconductor market. Given the mature and cost-effective technology based on silicon, the unification of Group III-V technologies with silicon technology has potential to provide a very good solution to the integration of semiconductor based electronics and photonics. General illumination consumes 19% of the world's total energy consumption. Due to a significantly increasing demand for energy-efficient technologies as a result of energy shortage and climate change, it is necessary to develop energy-efficient solid-state lighting sources based on white light emitting diodes (LEDs) in order to replace incandescent and fluorescent lights. Fabrication of white LEDs is mainly on III-nitride semiconductors. Major achievements achieved so far in the fields of III-nitrides are mainly limited to the growth on (0001) sapphire, the polar orientation. This generates a polarization issue, thus leading to piezoelectric electric fields. As a result devices exhibit a reduced overlap between the electron and hole wavefunctions, leading to a long radiative recombination time and thus low quantum efficiency.
The growth of GaN on silicon (i.e. GaN-on-Si technology) is coming up, but is also limited to polar c-plane GaN. The material issues which result from using the GaN-on-Si technology become even more severe compared with GaN-on-sapphire. Therefore, it is desirable to develop a new growth technology in order to achieve high crystal quality semi- or non-polar GaN on silicon, the most promising approaches to overcome the issue of the internal electric fields and thus achieving a step change in IQE.
In the last decade, several groups worldwide have devoted considerable effort to the development of semi/non polar GaN on silicon. However, the results are far from satisfactory due to a number of challenges which are heavily restricting development of semi/non polar GaN on silicon.
Unlike sapphire substrates, it is extremely difficult to obtain non/semi-polar GaN on any planar silicon substrate. So far, semi-polar GaN on silicon (including (11-22) and (1-101) orientations) can be obtained only through growth on patterned silicon substrates, for instance, anisotropy wet etching (113) silicon using KOH to fabricate a regular silicon pattern with inclined strips with a (1-11) facet which is at 58° to the surface of (113) silicon, where the GaN growth is performed on the (1-11) silicon facets selectively in order to form semi-polar GaN.
It is well-known that the growth of GaN on silicon needs to avoid a so-called “Ga melting-back” etching issue. This is due to a strong chemical reaction between the grown GaN and silicon, leading to a poor surface morphology and eventually growth collapse. For a planar silicon substrate, it is easy to sort out simply through an initial deposition of AlN buffer layer which can completely separate GaN layer from silicon substrate, such as polar c-plane GaN on (111) silicon. However, for the growth on the patterned (113) silicon with the inclined strips, unavoidably a large number of residual voids are generated during the growth, leaving the grown GaN to directly contact silicon (see FIG. 1). The melting-back etching increases with increasing growth temperature. The current solution is to reduce the growth temperature down to a point which is not accepted for GaN epitaxial growth in order to suppress the melting-back etching. As a result, the crystal quality is far from satisfactory.
A two-step selective growth method as illustrated in FIG. 1 was proposed by Amano at Nagoya (l T Murase, T Tanikawa, Y Honda, M Yamaguchi, H Amano and N Sawaki, Jpn. J. Appl. Phys. 50, 01AD04 (2011)). A triangle GaN stripe template was prepared on inclined (1-11) strip facets on (113) silicon using the same approach discussed above, followed by a deposition of a layer of SiO2 mask in order to selectively cover the triangle GaN stripes for further regrowth, where the regrowth will occur on the uncovered areas selectively. They achieved very impressive results. However, the approach had some problems. First, the melting-back issue has not yet been solved. Second, for the selective regrowth, the small (0001) face at the edge of the triangle GaN stripes needs to be carefully covered by SiO2 (dashed circle in FIG. 1) in order to avoid any growth along (0001). This has proved extremely difficult.